Mold Design

Ensuring proper resin requirements and applying reasonable fill pressure requirements can make all the difference in moldability.
Learn about Eastman's mold filling simulation to estimate required fill pressure and additional mold filling simulations that will predict fill patterns and volumetric shrinkage levels.
This section also covers important considerations for gate location including aesthetics and mechanical properties.

Graphics are provided to help you discover more "tricks of the trade" and avoid unnecessary injection molding challenges -- from defects in part appearance to costly design modifications after tooling construction.

Mold design

Moldability as well as product performance can be enhanced by proper part design features. Good design for moldability includes:

Providing reasonable flow length

Appropriate weld line location

Moderate injection pressure

Minimum clamp requirements

Minimum scrap rate

Easy part assembly

Minimal or no secondary operations, such as degating, painting, and drilling

Good design helps minimize:

Molded-in stress

Flash

Sink marks

Surface blemishes

Many other common molding defects that reduce quality or productivity

The ability to fill a mold with reasonable injection pressures is greatly influenced by the wall thickness of the part. Spiral flow data are helpful in choosing appropriate wall thickness. Gate location and wall thickness can be varied to achieve the best balance of part weight, clamp tonnage requirements, and weld line location.

Cooling

By designing parts so that they can be cooled properly, you can obtain lower cycle times and high quality parts while reducing cost.

Good cooling is absolutely critical when designing molds to run Eastman polymers.

Some effects of poor cooling:

Increased cycle time

Uneven cooling across parts or part to part

High levels of residual stress

Increased warpage

Sticking and difficulty in ejection

Although all are potentially serious problems, the most common difficulty when running Eastman polymers is sticking and difficulty in ejection.

Notes on cooling

Increasing the diameter of a cooling channel without maintaining velocity will result in a decrease in the total heat removed in a given channel. If turbulent flow is maintained, empirical correlations show that doubling the diameter while keeping flow volume (gpm) constant results in approximately 40% less heat transferred in spite of the fact that the area increases.

For turbulent flow, keeping the same coolant velocity while increasing the diameter of the cooling channel will theoretically provide a significant increase in heat transferred to a given flow channel. For example, if the diameter is doubled, the heat transferred should increase approximately 80%.

Sprue design

Proper sprue design is important for good molding and easy removal of the part from the mold. Sprue design for molds running Eastman polymers is important because:

Polyester materials tend to stick to tool steel when hot.

The sprue is so thick that it is the hottest and one of the most difficult areas to cool.

High-conductivity sprue bushing
Many Eastman customers are successfully using high-conductivity sprue bushings. The bushing is made from a high-conductivity copper alloy. It contains a hardened, 420 stainless steel nozzle seat to insulate from nozzle heat and for wear resistance. This is effective in reducing sprue sticking, increasing sprue rigidity for pickers and grabbers, and cutting cycle time. With this sprue bushing, a standard sprue taper of 42 mm/m (0.5 in./ft) has been found to be acceptable for good heat transfer.

Runner design

When designing runner systems, use the same guidelines that apply to most engineering polymers. The runners should be designed for smooth, fully balanced flow. Generously radiused transitions reduce material hang-up and shearing. Cold-slug wells are useful in trapping slugs of frozen material at the flow front. Vent the runners generously.

The size and appearance of the finished part must be considered in selecting the type and location of gates.

Hot runner systems

Design guidelines
Hot runner systems are common in applications using polyester materials. When properly designed, these systems can eliminate sprue and runner regrind, mold with lower pressures, reduce cycle times, and improve processing windows. The selection of a suitable hot runner system can vary greatly depending on the size of the part, polyester formulation, and part design. Therefore, it is critically important that runner design and selection be discussed jointly by the molder/end user, tool builder, hot runner supplier, and Eastman to arrive at the appropriate runner-system design to be used.

Uniform heating and good heat control
Excellent thermal control and good cooling at the gate is critical for molding polyester materials. The mold should be designed so that heat is quickly removed from the gate. This is best accomplished when the gate orifice is an integral part of the cavity steel rather than the hot runner system being an insert projecting through the cavity into the part. When the gate is in the cavity, cooling channels (drilled water lines or annular-shaped passages) can be incorporated to provide the cooling needed for the cavity in the gate area. Some hot runner suppliers offer gate-cooling inserts. Drooling, sticking, and stringing may occur if the gate does not cool properly. Steel that is directly heated as part of the hot drop should not contact the part directly; it should be insulated from the cooled portion of the mold.

We suggest separate cooling loops with individual flow and temperature control for cooling a hot drop gate. The additional control is very useful in debugging and optimizing gate appearance and performance.

Eliminating holdup spots
The flow channel for the plastic should be streamlined and uninterrupted. Any crevice or pocket where material can collect and degrade will probably cause defective parts.

Minimizing shear heating
The diameter of the flow path needs to be large enough to minimize the shear heating that can be caused by sharp corners or edges in the flow path at the gate or elsewhere. Mold-filling analyses can show shear heating and indicate potential problems during the design stage.

Valve gates
If possible, a valve system should be used when processing Eastman polymers. This has several advantages when compared with other hot-melt delivery systems. With valve gates, the melt channel is externally heated and the mechanical shutoff feature allows better control of gate vestige. The gate size is generally larger when compared with other available systems. The valve pin is retracted during the filling process, resulting in a less obstructed flow. The end result is less shear heating and pressure drop.

Venting and ejection
Venting allows gas replaced by the melt front to escape from the mold. Short shots, burning, and material degradation can occur if parts are not adequately vented. To prevent this:

Provide adequate venting in the proper location.

Check and clean vents regularly.

Use ejector pins as vents where possible.

Avoid vents that require mold disassembly for maintenance access.

Typical venting in molds designed for Eastman polymers
A good starting vent depth for molds designed to run Eastman polymers is 0.0005–0.001 in. (0.012–0.025 mm) for small parts or vents close to the gates and 0.001–0.0015 in. (0.025–0.038 mm) for larger parts. A typical land is 0.125–0.250 in. (3–6 mm) long and opens up into a larger channel that allows gas to vent from the mold.

Alloys for mold construction
There are several factors to consider when selecting steel for the mold:

Wear resistance

Toughness

Machinability

Polishability

Dimensional stability

Family molds
Family molds contain two or more cavities that mold different parts. Eastman polymers are being used successfully in family molds. Like any other polymer, the flow into the individual parts must be balanced. All parts should fill evenly and equally; otherwise, uneven packing will occur. In such cases, some parts will be overpacked and highly stressed, leading to warpage, and other parts will be underpacked or not completely filled.

Mold polishing and texturing

Mold polishing
Eastman polymers provide excellent gloss and pick up mold finish very well. Keep in mind that surfaces polished smoother than required for ejection only add to mold cost. In most cases, highly polished surfaces can hinder ejection if there is a vacuum drawn in low- or no-draft areas. Where no vacuum is drawn, polished surfaces generally eject better.

Texturing mold surfaces
Texturing is useful in hiding weld lines, flow marks, gate blush, sink marks, and scuffing. There are hundreds of standard patterns available. Basically, anything that can be drawn in black and white can be used as the basis for a texture pattern.

Draft angle guidelines
In most cases, a draft angle of 1 degree per side is suggested to aid ejection. However, ½ degree per side can be used to obtain reasonable dimensions in ribs, bosses, and other design features. Attention to the thickness at the top of ribs or bosses is needed to ensure structural strength.

Mold surface treatment to aid ejection
In some cases, a low draft angle may be required on a part, but the dimensions of the mold may not be suitable for proper cooling. Surface coatings or treatments that can aid in the ejection of parts are available. Eastman has completed an extensive study to determine which coatings and treatments are better release agents for our polyesters. The top three that we suggest follow.

FAQs

What is the preferred gating style for copolyester resins using injection molds equipped with hot runner systems?
Valve gates.

Are cold sprues an acceptable gating method for use with copolyester resins in injection molding tooling?
Yes. When cold sprues are the gating style of choice, a high-heat-transfer sprue bushing is recommended. Keep sprue length less than 3 inches. Provide cooling lines in close proximity. Use a slight press fit to ensure good thermal contact between the sprue bushing and surrounding tool steel.

What is a key design feature of injection molds constructed for copolyester resins?
Cooling. All steel surfaces within the cavity should be well cooled. If there are surface areas of the cavity that approach or exceed the glass transition temperature of the resin during rapid cycling, the resin can become sticky and difficult to eject.

What are common methods of cooling long cores in injection molds constructed for copolyester resins?
Bubblers, baffles, and spiral cooling channels are effective methods of cooling long cores. In cases where it is not possible to get cooling water close to the end of a long core, consider the use of high-heat-transfer alloys to increase thermal conductivity.

Why is cooling in the gate area important?
During the filling phase of injection molding, all of the heat going into the cavity passes through the gate, resulting in a high heat load in this area. Mold construction should focus on providing good thermal control in this region of the tool with either a cooling circuit in close proximity or a water-jacketed gate insert. It is often desirable to construct this gate-cooling circuit in such a way that it can be plumbed independently so that cooling in the gate area can be optimized independently of cavity cooling.